An evolutionary laboratory
Lake Sentani is a large, sprawling body of water covering over 9,360 hectares at the foot of the Cyclops Mountains. The lake sits just 73 meters above sea level and occupies a fault-controlled depression. Geologists believe it was once a saltwater arm of the sea, which tectonic uplift later isolated, trapping marine life. This isolation created a site for evolution. Over time, the trapped species adapted to the freshwater environment, and some diversified into a unique "species flock," a group of closely related species that evolved from a common ancestor within this confined habitat.
The most well-known members of this evolutionary shows were two species of rainbowfish: the Sentani rainbowfish (Chilatherina sentaniensis) and the red rainbowfish (Glossolepis incisus). These fish were endemic to the lake, meaning they were found nowhere else on Earth. The male red rainbowfish is brightly colored, developing a salmon-red coloration that can reach up to 12 cm in length. Alongside these, the lake also contained the now-extinct largetooth sawfish (Pristis microdon), a species that adapted to the freshwater conditions after being trapped. The last confirmed sighting of a sawfish in the lake was in 1974.
A fragile ecosystem collapses
The unique aquatic fauna of Lake Sentani faced a rapid decline in the latter half of the 20th century. A 1993 survey revealed that of 33 fish species present in the lake, 13 had been introduced from elsewhere. The introduction of invasive species, combined with pollution from the growing human population around the provincial capital of Jayapura, proved catastrophic for the endemic inhabitants. Introduced fish like tilapia competed for resources and preyed upon the eggs and young of the native rainbowfish.
The Sentani rainbowfish (C. sentaniensis) was particularly affected. Once common, it was last officially seen in 1954 and was thought to be extinct for decades. The IUCN lists the species as Critically Endangered, with a possibility of it being extinct in the wild. While some researchers reported rediscovering small populations in 2016, others are skeptical, suggesting these may be misidentified specimens of a related species. The red rainbowfish (G. incisus) is also listed as endangered in its native habitat. This species, however, has found a place in the aquarium trade and is bred in captivity by hobbyists around the world, creating a situation where it may soon exist only outside its natural environment.